Added consumer_clear()

include/mqtt/async_client.h

@@ -747,18 +747,39 @@ class async_client : public virtual iasync_client
     ) override;
     /**
      * Start consuming messages.
+     *
      * This initializes the client to receive messages through a queue that
      * can be read synchronously.
+     *
+     * Normally this should be called _before_ connecting the client to the
+     * broker, in order to have the consumer queue in place in the event
+     * that the immediately starts sending messages (such as any retained
+     * messages) while the client is still in the context of the connect
+     * call.
+     *
+     * This _must_ also be called before calling any 'consume_message' or
+     * "'consume_event' methods.
+     *
+     * Internally, this just creates a thread-safe queue for `mqtt::event`
+     * objects, then hooks into the message and state-change callback to
+     * push events into the queue in the order received.
      */
     void start_consuming() override;
     /**
      * Stop consuming messages.
+     *
      * This shuts down the internal callback and closes the internal
      * consumer queue. Any remaining messages and events can be read until
      * the queue is emptied, but nothing further will be added to it.
      * This will also wake up any thread waiting on the queue.
      */
     void stop_consuming() override;
+    /**
+     * This clears the consumer queue, discarding any pending event.
+     */
+    void clear_consumer() override {
+        if (que_) que_->clear();
+    }
     /**
      * Determines if the consumer queue has been closed.
      * Once closed, any events in the queue can still be read, but no new
@@ -884,7 +905,6 @@ class async_client : public virtual iasync_client
         this->try_consume_message_until(&msg, absTime);
         return msg;
     }
-
     /**
      * Read the next message from the queue.
      * This blocks until a new message arrives.

include/mqtt/iasync_client.h

@@ -449,6 +449,10 @@ class iasync_client
      * messages.
      */
     virtual void stop_consuming() = 0;
+    /**
+     * This clears the consumer queue, discarding any pending event.
+     */
+    virtual void clear_consumer() {}
     /**
      * Determines if the consumer queue has been closed.
      * Once closed, any events in the queue can still be read, but no new

include/mqtt/thread_queue.h

@@ -202,6 +202,17 @@ class thread_queue
         guard g{lock_};
         return is_done();
     }
+    /**
+     * Clear the contents of the queue.
+     * This discards all items in the queue.
+     */
+    void clear() {
+        unique_guard g{lock_};
+        while (!que_.empty())
+            que_.pop();
+        g.unlock();
+        notFullCond_.notify_all();
+    }
     /**
      * Put an item into the queue.
      * If the queue is full, this will block the caller until items are